1. Field of the Invention
The present invention relates to a laminated band pass filter including a plurality of dielectric layers and a plurality of electrode layers laminated on one another.
2. Description of the Related Art
In general, a laminated band pass filter is used as a high-frequency band pass filter. The laminated band pass filter is configured such that a plurality of LC resonators are included in a laminated body constituted by laminating dielectric layers and electrode layers. The laminated band pass filter thus configured is suitable for miniaturization and cost reduction.
Conventional laminated band pass filters are disclosed in Japanese Unexamined Patent Application Publication No. 4-6911, Japanese Unexamined Patent Application Publication No. 2000-201001, Japanese Unexamined Patent Application Publication No. 2003-198226, and International Publication No. 02/009225.
FIG. 1 is a circuit diagram illustrating a laminated band pass filter disclosed in Japanese Unexamined Patent Application Publication No. 4-6911.
This filter includes a plurality of LC parallel resonant circuits which have coils and capacitors, each of the coils and a corresponding one of the capacitors being connected to each other. The LC parallel resonant circuits are magnetically connected to the adjacent LC parallel resonant circuits by the coils of resonators adjacent to one another.
FIG. 2 is a sectional view illustrating the laminated band pass filter.
A ground electrode 11 is arranged in a first layer 10-1. Capacitor electrodes 12 and coil patterns 13 are arranged in second and third layers 10-2 and 10-3. Capacitance is interposed between the ground electrode 11 and the capacitor electrode 12. The coil patterns 13 arranged in two layers are electrically connected to one another through blind through holes 14. A LC parallel resonant circuit is configured by the ground electrode 11, the capacitor electrodes 12, the coil patterns 13, and the blind through holes 14. An LC parallel resonant circuit is further laminated in layers under a fourth layer 10-4. As a result, a plurality of LC parallel resonant circuits are magnetically connected to one another through the coils.
A laminated band pass filter disclosed according to Japanese Unexamined Patent Application Publication No. 2000-201001 includes a plurality of LC resonators having self-resonant capacitors. The LC resonators are laminated in a thickness direction of a laminated body, and are electromagnetically connected to one another. As a result, components can be miniaturized while physical distances among the LC resonators required due to design of the band pass filter are ensured.
A laminated band pass filter disclosed in Japanese Unexamined Patent Application Publication No. 2003-198226 includes filter lines, each of the filter lines having a pair of lines, which are parallel to each other and which are arranged on portions of wiring layers. The first and second filter lines are arranged on different circuit layers and face each other in parallel. The lines which face each other in a thickness direction are electrically connected to each other at a terminal portion and bent through a dielectric layer.
The laminated band pass filter disclosed in International Publication No. 02/009225 has two strip lines constituting a resonator which are arranged in the same layer with a predetermined interval.
According to the laminated band pass filter of Japanese Unexamined Patent Application Publication No. 4-6911, the coils included in the LC parallel resonators are formed in a two-layer coil pattern. Therefore, magnetic connection among the LC parallel resonators is likely to become considerably strong. Furthermore, since the coils are formed in the two-layer coil pattern, there arises a problem in that insertion loss of the laminated band pass filter becomes large due to deterioration of Q values of the coils. To address these problems, sufficient intervals are required among the LC parallel resonators. However, in this case, the thickness and the size of the laminated band pass filter become large.
According to the laminated band pass filter of Japanese Unexamined Patent Application Publication No. 2000-201001, the capacitors spontaneously resonate. Therefore, each of the LC resonators is configured by a capacitance component of a capacitor electrode and an inductance component included in the capacitor electrode. In this case, it is difficult to configure a resonator having a desired inductance, and there arises a problem in that a low-loss characteristic of the band pass filter is not obtained.
According to the laminated band pass filters of Japanese Unexamined Patent Application Publication No. 4-6911 and International Publication No. 02/009225, small and low-loss band pass filters can be attained. However, in order to attain a characteristic of steep attenuation from a pass band to out-of-band, resonators should be arranged in multiple stages and the filters arranged in the two stages should be laminated in a lamination direction and connected to one another. Therefore, there arises a problem in that, when a filter having multiple stages is configured, the thickness thereof becomes large.
Furthermore, in the conventional laminated band pass filters, in a case where LC parallel resonators having capacitor electrodes and inductor electrodes are arranged in a laminated body, and the inductor electrodes adjacent to one another are inductively coupled with one another, a ripple (deviation) may occur in a band pass characteristic of a pass band.